1. Field of the Invention
The present invention relates to systems and methods for transmitting data, and in particular to a system and method for using equalization to restore nonlinearity for full traveling wave tube amplifier curve measurements.
2. Description of the Related Art
Digital signal communication systems have been used in various fields, including digital TV signal transmission, either terrestrial or satellite. As the various digital signal communication systems and services evolve, there is a burgeoning demand for increased data throughput and added services. However, it is more difficult to implement either improvement in old systems or new services when it is necessary to replace existing legacy hardware, such as transmitters and receivers. New systems and services are advantaged when they can utilize existing legacy hardware. In the realm of wireless communications, this principle is further highlighted by the limited availability of electromagnetic spectrum. Thus, it is not possible (or at least not practical) to merely transmit enhanced or additional data at a new frequency.
The conventional method of increasing spectral capacity is to move to a higher-order modulation, such as from quadrature phase shift keying (QPSK) to eight phase shift keying (8 PSK) or sixteen quadrature amplitude modulation (16 QAM). Unfortunately, QPSK receivers cannot demodulate conventional 8 PSK or 16 QAM signals. As a result, legacy customers with QPSK receivers must upgrade their receivers in order to continue to receive any signals transmitted with an 8 PSK or 16 QAM modulation.
It is advantageous for systems and methods of transmitting signals to accommodate enhanced and increased data throughput without requiring additional frequency. In addition, it is advantageous for enhanced and increased throughput signals for new receivers to be backwards compatible with legacy receivers. There is further an advantage for systems and methods which allow transmission signals to be upgraded from a source separate from the legacy transmitter.
It has been proposed that a layered modulation signal, transmitting non-coherently both upper and lower layer signals, can be employed to meet these needs. Such layered modulation systems allow higher information throughput with backwards compatibility. However, even when backward compatibility is not required (such as with an entirely new system), layered modulation can still be advantageous because it requires a traveling wave tube amplifier (TWTA) peak power significantly lower than that for a conventional 8 PSK or 16 QAM modulation format for a given throughput.
To provide a layered modulation scheme (as described in detail below), a reconstructed upper layer signal is subtracted from a received composite signal to reveal a lower layer signal. As such, the lower-layer signal performance is impacted by how closely the upper-layer signal can be reconstructed relative to the original signal. In other words, the lower layer signal performance is impacted by the fidelity of the reconstructed signal. Thus, layered modulation requires clean cancellation of the upper-layer signal to expose the lower-layer signal for further processing. Clean cancellation requires TWTA non-linearity/distortion to be accurately reproduced in the reconstruction of the upper-layer signal. On-line estimation of the required TWTA characteristics is imperative in minimizing the required TWTA power and the complexity of satellite operation. In addition, the non-linearity measurement may be used to monitor the health of satellite TWTAs and perform other communications diagnostics. However, such an accurate reproduction and knowledge of the TWTA non-linearity presents a significant roadblock.
With a TWTA, there is a region of approximate linearity, in which the output power is nearly proportional to the input power, followed by a curved transition to a point where the output power levels off and reaches a maximum. At this point (i.e., when the TWTA curve is well in the non-linear region), the amplifier is said to have reached saturation. Due to this non-linearity and to avoid intermodulation, the input power is often “backed off” by a particular amount (e.g., 6 dB). The resulting point on the curve after the input power is “backed off” is referred to as the operating point of the TWTA. When subsequently reconstructing the upper layer signal, the amount of distortion/non-linearity used to create the original signal serves to increase the fidelity of the reconstructed signal. Thus, to produce a high fidelity reconstructed upper layer signal, knowledge of the non-linearity (and the operating point) is important. Accordingly, the inclusion of (or taking into account) TWTA non-linearity may improve upper-layer signal cancellation ratio by 10 dB or more (the cancellation ratio between non-linearity-induced noise before and after cancellation is improved). However, linear distortions in a satellite communication channel may block and/or obscure a part of the TWTA nonlinearity visible in the process of TWTA nonlinearity measurement.
Errors in the estimation of the operating point can have a significant impact when reconstructing the upper layer-signal. The impact of amplitude (AM-AM [amplitude modulation to amplitude modulation]) and phase (AM-PM [amplitude modulation to phase modulation]) nonlinearity due to operating point errors may be individually analyzed based on shift analysis. Individual impacts may then be combined for total impact. To evaluate performance impacts, the synthesis of a layer-modulated signal with known TWTA non-linearity and system/representative operating CNR (carrier to noise ratio) may be used. The upper-layer cancellation error may then be calculated for each amount of simulated operating point error in the signal reconstruction process. Thus, the upper layer cancellation ratio may be plotted against the operating point displacement. The cancellation error can then be converted into an amount of lower-layer CNR degradation, which increases the CNR required for signals of both upper and lower layers. Such an increased CNR illustrates the impact of operating point estimation errors.
FIGS. 14A and 14B illustrate the impact of operating point errors in signal reconstruction with an example AM-AM and AM-PM nonlinearity. In FIGS. 14A and 14B, the sensitivity of signal reconstruction error is plotted against the TWTA input operating point error. The effective noise is calculated as a measure of signal reconstruction error.
In FIG. 14A, a set of generic TWTA non-linearity curves are used. The signal reconstruction process is assumed to have full knowledge about the non-linearity curves but is otherwise uncertain about the operating point. The performance plots of FIG. 14A indicate that cancellation errors are below −25 dB for an input operating point error up to about +/−1 dB.
In FIG. 14B, the performance plots are based on the same TWTA non-linearity but with an input backoff of 8 dB. With such an input backoff, the linearity is improved and is less susceptible to TWTA operating point error. As a result, reconstruction and cancellation errors are greatly reduced as indicated in FIG. 14B. The effective noise is below −33 dB with the same input operating error up to about +/−1 dB as in FIG. 14A.
Accordingly, there is a need for systems and methods for implementing layered modulation systems that accurately determine TWTA non-linearity and the operating point.
In the prior art, TWTA non-linearity measurements are performed on the ground before a satellite is launched. The TWTA operating point is then obtained from telemetry tracking and control (TT&C) commands that set the operating point of the TWTA (the procedure assumes that TWTA characteristics have little changed since the satellite was launched). In other words, the operating point set by TT&C commands during pre-launch measurements is used post-launch after receiving the signals from the satellite. However, TWTA characteristics including the non-linearity and the effective and operating point may change over time (including after satellite launch) and temperature. In this regard, the upper-layer signal cancellation in layered modulation may not be as accurately done without non-linearity updates.
Another prior art method is to estimate the non-linearity by trial and error, hoping to converge to a required accuracy within a reasonable effort and cost.
Accordingly, what is needed is a system and method for accurately determining the non-linearity of a TWTA as it changes over time/temperature. Further, what is desired is the capability to make such determinations at any time, from anywhere within the satellite downlink footprint, automatically and accurately. The present invention meets this need and provides further advantages as detailed hereafter.